1. Field of the Invention
This invention relates to an in-situ combustion process for the cyclic thermal stimulation of heavy oil around a producing well wherein oxygen or a fluid containing a minimum of about 75% by volume pure oxygen is used as the oxidant so as to react with the oil to release heat of combustion and to produce high concentrations of carbon dioxide. The increased temperature, pressure, and the dissolution of the CO.sub.2 in the reservoir oil reduces its viscosity and thereby increases oil production from the well when it is returned to production.
2. Description of the Prior Art
Repetitive stimulation of oil producing wells is a production practice of long standing. The phrase "cyclic stimulation" is often used to reflect anticipated production rate increases, the duration of which is relatively short as compared to the total life of the well. The cause of the production increase arises from either (1) an increase in pressure driving reservoir fluids toward the producing well, or (2) a decrease in resistance to flow of the fluids such as reduction in viscosity or removal of impediments to flow in the reservoir rock surrounding the well. The viscosity reduction may be achieved through use of a low viscosity fluid solvent and by increasing the temperature of the reservoir fluids and rock in the proximity of the reservoir.
In wells producing heavy (viscous) oil, cyclic thermal stimulation has become widespread in use. Two somewhat different thermal stimulation techniques have been developed: (1) cyclic steam injection, and (2) cyclic in-situ combustion. A typical cyclic steam stimulation may include: (1) injection of steam into a producing interval for a period that may extend up to several weeks, depending on thickness of the reservoir, well spacing, rate of steam injection, etc.; (2) allowing a "soak" period (which in some circumstances is not necessary); and (3) returning the well to production. The heat introduced into the reservoir rock continues to be effective for some time in warming and reducing the viscosity of the oil, thus increasing the production rate. The effects of the stimulation will decline over a period of a few months whereupon the treatment may be repeated.
Instead of using fuel-fed steam generators, cyclic in-situ combustion may be used to heat the reservoir. With this technique, air is injected into the reservoir through the producing well, which, after ignition, burns a small portion of the crude oil "in-situ", generating heat which is conveyed outward from the well into the surrounding reservoir by the flue gas formed and by vaporized crude oil and water. Water may be injected along with, intermittently, or following air injection to form steam and hot water which will convey the released heat of combustion farther into the reservoir. Although this method of stimulation may utilize fuel of less value than the steam process, wherein the steam is generated prior to injection into the reservoir, use of the latter process is generally favored. One major disadvantage of the combustion method is the requirement of compressing to injection pressure approximately four mols of nitrogen for every single mol of oxygen in air to support the combustion reaction. This increases cost and also dilutes the carbon dioxide concentration in the flue gas, greatly diminishing its efficacy as a solvent gas for reducing viscosity of the heavy oil.
The method of this invention is a major improvement in the combustion stimulation technique in that it uses oxygen or a fluid containing a minimum of about 75% by volume pure oxygen as the oxidant injected into the reservoir through the production well. The cycle of the process would be similar to that used with air: i.e., (1) inject the oxidant, which after ignition causes movement of a burn front through the reservoir rock surrounding the well; (2) allow a "soak" period (which is optional); and (3) return the well to production. The latter step usually requires installation of a downhole pump to remove produced liquids from the well.
The advantages resulting from the use of oxygen or a fluid containing a minimum of about 75% by volume pure oxygen include:
1. Elimination of large amounts of "inert" gas, i.e., nitrogen, which is costly to compress for injection. Also the presence of the inert nitrogen gas as a separate phase in the pores of the reservoir rock impedes the flow of oil toward the well.
2. The concentration (and partial pressure) of the CO.sub.2 formed in the combustion reaction is increased, and correspondingly its solubility in the heavy oil is increased. As a result, the viscosity of the heavy oil containing larger amounts of solvent gas is substantially reduced, and oil production rate is increased accordingly.
3. The increased CO.sub.2 content in the oil phase increases the extent to which the "solution gas drive" can contribute to the displacement of oil toward the production well.
4. Ignition of the combustion reaction in-situ is facilitated by the higher oxygen concentration of the injected gas. "Auto-ignition" will occur with a greater number of crude oils, thus reducing the need to use downhole burners, electric heaters, or steam preheating to start the combustion reaction. (This does not preclude the use of any of these methods where the crude oil properties do not favor auto ignition.)
5. Water injection along with or intermittent to the injected oxidant may be used as in "wet combustion" using air and water. The advantages of increased heat transport farther into the reservoir by the steam formed in-situ from heat released by the combustion reaction also apply with oxygen or enriched air combustion. The increased solubility of CO.sub.2 in the condensed water also enhances its expulsion from the reservoir to the producing well which also enhances the displacement of the heavy oil toward the producing well.
In U.S. Pat. No. 3,174,543 to Sharp there is described a method of recovering oil by producing carbon dioxide in the reservoir region surrounding an injection well by in-situ combustion and then introducing water into the reservoir to drive the carbon dioxide through the reservoir to displace the reservoir oil toward a production well. The present process is an in-situ combustion stimulation process that takes place in the reservoir immediately surrounding the bottom of a producing well using oxygen or a fluid containing a minimum of about 75% by volume pure oxygen as the oxidizing medium which results in the formation of a combustion gas comprising a high concentration of carbon dioxide. The carbon dioxide readily dissolves in the oil and reduces its viscosity. The heat generated in the reservoir by combustion also reduces the viscosity of the oil phase thus improving its flow through the formation when production is resumed. By the process of this invention therefore, a more effective recovery of the heavy crude oil is obtained.
Thermal oil stimulation processes using the so-called "huff-n-puff" gas injection techniques are disclosed in U.S. Pat. Nos. 3,332,482 to Trantham, 3,369,604 to Black et al. and 3,465,822 to Klein.
U.S. Patent to Trantham, 3,332,482, discloses a process for the secondary recovery of viscous oil using an in-situ combustion process at the bottom of a producing well in which air is used as the oxidizing medium. In this process, air is injected into the production well and the oil surrounding the bottom of the well is ignited to establish a combustion zone. Combustion is continued until the reservoir is plugged by viscous oil which results in a substantial increase in pressure. Combustion is terminated and the well is opened for production so that the compressed gases within the reservoir remote from the production well and beyond the plugged area drive the oil into the hot burned-out area between the plugged area and the production well where it is heated, perhaps upgraded somewhat, and finally recovered through the production well. Inherent in this process is the production of a gas, which is normally referred to as flue gas, which gas is composed predominantly of nitrogen and lesser amounts of carbon dioxide, carbon monoxide and other gases derived from the crude oil. The carbon dioxide in the flue gas is diluted by the nitrogen and other gases and is much less soluble in the reservoir oil than a gas consisting of substantially pure carbon dioxide or a gas containing a higher concentration of carbon dioxide than the flue gas produced by the use of air as the oxidizing medium. The solubility in reservoir oil of carbon dioxide formed with air combustion, at a given pressure, may be five to ten times less than that formed from oxygen combustion.
U.S. Pat. No. 3,369,604 discloses a method for stimulating producing wells using a combination of steam stimulation and in-situ combustion wherein air, or a mixture of air and oxygen is used as the oxidizing gas.
U.S. Pat. No. 3,465,822 to Klein, discloses a thermal oil stimulation process in which in-situ combustion is initiated around a well by air injection followed by injection of water and injection of inert gas, sequentially, and thereafter opening up the well to flow of fluids, including oil.
Also, in a Society of Petroleum Engineer of AIME article, SPE 9228, presented on Sept. 23-26, 1979, in Las Vegas, Nev., entitled "A Parametric Study of the CO.sub.2 HUF-n-PUF Process" there is disclosed the results of Mathematical Model studies of the use of carbon dioxide as a solvent gas in cyclic well stimulation. The carbon dioxide is not prepared in the well by in-situ combustion as in the present process and offers no advantages associated with the heat generated by oxygen combustion of the reservoir oil.
None of the prior art discloses the improved method of recovering oil around a well using in-situ combustion stimulation wherein the oxidizing medium is oxygen or a fluid containing a minimum of about 75% by volume pure oxygen so as to produce increased concentrations (and partial pressures) of carbon dioxide in the combustion gases. The carbon dioxide dissolves in the reservoir oil reducing its viscosity, thereby facilitating its flow to the production. The viscosity of the reservoir oil is further reduced by the heat generated in the reservoir by combustion.